Dissolve yeast in warm water. In a large bowl combine milk, sugar, and salt. Use a mixer to beat in molasses, butter, yeast mixture and 1 cup of rye flour. Use a wooden spoon to mix in the remaining rye flour. Add white flour by stirring until the dough is stiff enough to knead. Knead 5 to 10 minutes, adding flour as needed. If the dough sticks to your hands or the board add more flour. Cover dough and let rise 1 1.5 hours or until double. Punch down dough and divide to form 2 round loaves. Let loaves rise on a greased baking sheet until double, about 1.5 hours. Preheat oven to 190ºC . Bake for 30 to 35 minutes. Makes 2 round loaves.